Abstract: Phytochromes are photochromic proteins that exist with 2 photo-interconvertible isomeric forms: the red-light-absorbing form (Pr) and the far-red-light-absorbing form(Pfr). A short exposure to light in the middle of the night causes inhibition or promotion of growth and flowering in plants, namely night break. After red light night break treatment, phytochrome will convert from Pr form to Pfr form to influence tomato growth, flowering and yield. Therefore night break has been extensively applied as a tool to study the photoperiodic control of flowering and growth for many years. In this study, Jinpeng NO.1 tomato seedlings were used as the material. After germination, the same-sized plants were selected and transplanted to the light emitting diode(LED) growth chamber with a temperature of 28 ℃ during the day, and 20 ℃ temperature, 12 h photoperiod and 65% relative humidity during the night. The daily night break treatment was started from 20:00 to 08:00 by red LED light lasting for 5 min at an intensity of 20 μmol/(m2·s). The night break treatments were applied in different frequencies, including 2 times (once every 4 h), 3 times(once every 3 h), 5 times (once every 2 h), 11 times (once every 1 h), and constant red light in night. The effects of night break by red LED light on tomato seedling morphology, flowering and hormone contents were studied. The Michaelis-Menten growth model was used to fit the relationship between night break times and the stem elongation, and stem diameter respectively. When all tomato seedlings flowered, 20 tomato seedlings in each treatment were transplanted in a greenhouse, to study the influence on per tomato fresh mass and per plant yield when tomato seedlings were pretreated by red light night break. The results showed that, the night break frequencies had significant effect on tomato stem diameter and stem height. When the night break time was 5, the stem diameter decreased by 25.08% and the stem height increased by 30.72% compared with the control. The days needed for flowering were also significantly influenced by night break frequencies. When the night break time increased to 5, the days needed for flowering increased from 39 to 53 d, and the leaves needed for flowering increased from 8 to 11 leaves. Dry matter of the whole tomato plant showed no difference between the red light treatments at night and the control, but the distribution of carbohydrate in different organs changed significantly. When the night break was 5, 11 times or constant red light at night, the stem dry matter significantly decreased by 28.30%, 30.19% and 33.96% respectively compared with the control; on the contrary, leaf dry matter increased by 16.67%, 19.44% and 18.06% respectively. The contents of indole acetic acid (IAA) and gibberellin(GA₃) were also significantly influenced by night break frequencies. When the night break time was 5, the content of IAA in stem and leaf decreased significantly by 51.10% and 29.93% respectively, and the content of GA₃ in stem and leaf decreased significant by 46.71% and 40.47% respectively. The tomato plant’s pretreatment by red light at night significantly influenced tomato later growth and yield. When the night break time was 5, the per fresh fruit mass of the first cluster increased by 41.78% and per plant fresh fruit mass increased by 11.36% compared with the control. According to the results of red light treatment at night, we can control the growth and development of tomato plants by controlling the night break frequencies.